Interference pigments for effect paint, paint manufactured therefrom, and paintwork applied therewith

ABSTRACT

The invention relates to platelet-shaped interference pigments for effect paints for painting objects of daily use, especially vehicle bodies, with the interference pigments including liquid crystal polymers (LCP) with main and/or side groups--mesogens--which are arranged at least approximately chirally-nematically or the like. In order to be able to paint different colored backgrounds in a covering fashion using the transparent interference pigments, the interference pigments are built up in several layers and also have at least one and preferably two interfering layer(s) of liquid crystal polymers and even of a single, preferably centrally located, light-absorbent layer, with the light-absorbent layer absorbing at least a portion of the visible light spectrum. Advantageously the light-absorbent layer can absorb light in the entire visible light spectrum, in other words appear black. It is also possible for it to be made light-absorbent within those partial areas of the visible light spectrum that lie outside one of the colors of the interference pigments. A method is also disclosed for manufacturing the multilayer interference pigments.

BACKGROUND OF THE INVENTION

This invention relates to interference pigments, a method formanufacture thereof, effect paint formulated with interference pigments,and paintwork applied therewith.

This application claims the priority of German patent application No.19619973.5-43, filed May 17, 1996, the disclosure of which is expresslyincorporated by reference herein.

In the effort to obtain brilliant or effect-filled color impressions,especially in motor vehicle bodies, so-called effect paints have beendeveloped. EP 383 376 Al describes such an effect paint in which smallmica platelets are coated uniformly on all sides with a cross-linkedliquid crystalline polymer (LCP) in a chiral-nematic arrangement of themesogens. The color appearance of such pigments results from aninterference phenomenon. Those light waves in the incident light whosewavelengths interfere with the equidistant lattice plane spacing of theliquid crystalline polymer are reflected from the LCP layer, while lightcomponents of other wavelengths pass through the transparent paint body,and are partly reflected from the surface of the mica platelets andpartly absorbed. Since light absorption is only slight because of thetransparency of the mica platelets, the interferential color effectobtained with this type of interference pigment is determined primarilyby the absorption effect of the primer; more detail on this is providedbelow. In addition, uniform and complete coating of small mica particleswith an LCP coating without the pigments clumping in the process is veryexpensive, making effect paint and therefore effect paintwork on theobject in question quite expensive.

For example, the manufacture of interference pigments and liquid crystalpolymers is known from EP 601 483 B1 (corresponding to DE 42 40 743 A1),DE 44 18 075 A1, or DE 44 18 076 A1, these polymers creating a moreintensive interferential color effect and also being easier tomanufacture. These interference pigments consist as a whole of smallpieces of a thin, crosslinked, colorless, clear transparent film ofliquid crystalline polymers. The color effect that can be achieved withthem is based on the regular structure and uniform arrangement of themolecules in the shape of a liquid crystal and on the resultantinterference with a certain portion of the light spectrum in which thepigment produces a reflecting effect. The other light components passthrough the pigment. As a result, striking color effects of differentkinds can be produced, depending on the structure of the paintworkand/or the interference pigments and their mixtures. The basic shade ofthe interference pigments is determined by the color impression, or thecolor that is seen when the painted surface is illuminatedperpendicularly and viewed perpendicularly. When the beam path isdirected at an angle to the surface, the lattice plane intervals, due tothe geometry, appear to be changed relative to the orthogonal beamdirection, so that the color impression shifts toward another color thatis offset toward shorter wavelengths in the color spectrum, and thisdepends on the relative viewing direction of the surface. Depending onthe position of a certain part of the surface relative to the line ofsight of the viewer, the surface portion appears in the basic color orin the other color with the shorter wavelength. For example,interference pigments with a basic red color can change to green; acolor change between green and blue can be produced using differentinterference pigments.

The intensity of the colors that can thus be perceived increases withthe darkness of the background color on which the paint layer thatdetermines the color rests, with the color of the background howeverhaving to be determined by color pigments that have an absorptiveeffect. This is due to the fact that the light components that passthrough the interference pigments are absorbed more or less completelyby the dark background to a degree that increases with the darkness ofthe background.

The interference pigments of the type under discussion here are obtainedby the polymers being spread in the liquid or liquid crystalline stateonto a smooth substrate, a polished roller for example, forming a thinfilm. The spreading process produces an alignment of the moleculeswithin the film, namely a homogeneous orientation; it is only because ofthis alignment that the color exhibits an interference color. During theshearing of the liquid crystalline polymers, equidistant lattice planesautomatically form during spreading and thus create color-selectivedeflecting structures.

Known interference pigments, as we have said, are clear and transparentand therefore do not have any covering power. They therefore require auniformly colored background in order to produce a uniform colorimpression. On the other hand, the color of the background can changewithin a wide range of workpieces, for example when filler paint fromdifferent manufacturers is used, with said paints differing in color, orwhen differently primed structural or body parts are processed on oneand the same workpiece. Prior to the application of an interferenceeffect paint that has no covering power, therefore, assurance must beprovided that not only all of the parts of the workpiece but also all ofthe workpieces within a series are painted uniformly with only oneprimer whose color is specified, necessitating an additional paintingand baking process. In the case of repairs as well, the body part whosepaint is to be repaired or even the entire body must first be primeduniformly and in exactly the same color as originally and this primermust then be baked. Only then can the effect paint be applied to therepair. This is costly and awkward for routine applications butespecially for repairs. Theoretically, it would be possible whenformulating an effect paint to mix absorption pigments into the paintalong with interference pigments in order to make the paint exhibitcovering power as a result. However, to achieve a good covering power,such a large amount of absorption pigment would have to be added to thepaint that the effect of the interference pigments would be very sharplyattenuated as a result because they are dependent on good transparencyof the body of the paint. If only a small amount of absorption pigmentsis mixed in with the paint and adversely affects the interferentialcolor effect of the interference pigments only slightly, the paint, inorder to achieve a sufficient covering power, would have to be appliedin a layer that was so thick that it could not be applied in a singlestep. Such a paint would have to be applied in a minimum of two steps,which would offer no advantages over a previous application of a primerwith good covering power.

SUMMARY OF THE INVENTION

The primary object of the present invention is to improve interferencepigments in such fashion that they can be manufactured in simple fashionand have a powerful effect in terms of their interferential colorphenomena, yet can be formulated with a paint that likewise has goodcovering power.

This object is achieved according to the invention with respect to thepigments by providing platelet-shaped, multilayered interferencepigments for effect paint for painting objects of daily use, especiallyvehicle bodies, with at least one layer of interference pigmentscomprising liquid-crystal polymers (LCP) whose mesogens are arranged atleast approximately chirally-nematically and/or smectically and/orcholesterically; with a light-absorbent layer also being provided in theinterference pigments, with the layer absorbing at least a part of thevisible light spectrum; and with the edges of the platelet-shapedinterference pigments being made in the form of circumferentiallyuncoated fractured edges of multilayer sandwich.

With respect to the manufacturing process the object is achieved in twoways, namely by applying liquid crystalline polymers (LCP) in the liquidstate as a thin film to a smooth substrate and as a result arranging themesogens are at least approximately chirally-nematically and/orsmectically and/or cholesterically, curing this film and processing thecured film into pigments. Following curing of a first film, the latteris initially left on the smooth substrate and a layer of a lightabsorbent dye is applied to this film in a layer thickness that producesabsorption and is then cured. Only then is this two-layer cured filmpulled off the substrate and fractured into platelet-shaped particles.

Alternatively, a film with a light-absorbent dye is applied as asubstrate in a layer thickness that produces absorption, onto which filmthe film made of liquid crystal polymers (LCP) is applied and cured inthe liquid state, then this sandwich composed of the absorption film andcured LCP film is fractured into platelet-shaped particles.

By the application of a film of liquid crystal polymers on a smooth,geometrically defined substrate, a preproduct for interference pigmentscan be produced in an economical fashion that produces a goodinterferential color effect. According to this method, a multilayersandwich with a light-absorbing covering or intermediate layer can beproduced on or in the film of pigment preproduct that lends the film, orthe interference pigments produced therefrom, the desired coveringpower. Absorption pigments, in order to achieve a covering effect, donot have to be mixed with an effect paint that is formulated with theinterference pigments according to the invention. On the other hand, thepaint or color designer is of course free to add absorption pigments ofcertain colors to an effect paint to achieve further desired coloreffects. When, during application, a mechanically supported shearingeffect is exerted on the applied liquid crystalline polymers, forexample by spreading, because of the rapid and especially good alignmentand uniform arrangement of the mesogens in the film, a very goodinterferential color effect is achieved.

It is to be understood that the word "paint" as used herein includeslacquer and any other covering composition which would or could be usedfor the same purpose.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a body part with a cross section,enlarged in portions, through the wall of the part and the paint;

FIG. 2 is a representation of a three-layer interference pigment; and

FIG. 3 is a representation of a two-layer interference pigment.

Attention is directed to FIG. 1 which shows a body part 1 schematicallyrepresented by a dot-dash line and in portions thereof, an enlargedcross section through sheet metal 2 and multilayer paintwork 3, with thepaintwork shown as an example consisting of a primer 4, filler paintwork5, effect paintwork 6, and a clear paint layer 7.

Platelet-shaped interference pigments 8 applied in three layers (FIG. 2)or two-layer interference pigments 9 (FIG. 3) are embedded in thecolor-determining paintwork layer of effect paint 6. Regardless of thenumber of layers of interference pigments, there is at least one layer14 composed of liquid crystalline polymers (LCP) with main and/or sidegroup mesogens that are arranged at least approximatelychirally-nematically and/or smectically and/or choleristically. Forexample, regular helices 10 are formed in the liquid crystallinepolymers which, because of their regular structure, correspond to a sizeof a picture element in the vicinity of the light wave length of acertain color of visible light and as a result produce theinterferential color effect, with the color also depending on theviewing angle. The platelet-shaped interference pigments have athickness (S) of about 30 to 50 μm and a diameter (L) of 5 to 100 μm,preferably 10 to 50 μm.

In order for interference pigments 8 or 9 to be simple to manufactureand to be highly effective in terms of their interferential colorphenomena, yet capable of being formulated with paint with equally goodcovering power, a light absorbent layer 11 is incorporated into theinterference pigments that absorbs at least a part of the visible lightspectrum. The edges of the platelet-shaped interference pigments aremade as circumferentially uncoated fractured edges 12 of multilayer 13or 13'.

The three-layer interference pigments 8, preferably structuredsymmetrically with respect to the middle layer according to FIG. 2, withlight absorbent layer 11 located centrally in sandwich 13, do have agreater manufacturing cost than two-layer interference pigments 9according to FIG. 3, but the three-layer pigments have the advantage ofproducing their interferential color effects on both the top and bottom,in other words in the two possible relative positions within the paintlayer, while the interference pigments that consist of two layers andare easier to produce exhibit this effect on only one side. According toprobability, about 50% of the interference pigment 9 embedded in paintlayer 6 has the interferentially effective LCP side outward while theother half has the absorption side 11 outward. This produces a lessbrilliant color phenomenon of the effect color by comparison with theuse of three-layer interference pigments.

In general, a substance is chosen for light absorbent layer 11 that hasa light absorbing effect throughout the entire visible light spectrum,in other words it appears black. As a result, the respective effectcolors, regardless of which they are, appear clearly. It is alsopossible to use instead a substance for light-absorbent layer 11 that islight-absorbent only within a narrow partial area of the visible lightspectrum, which for example coincides with one of the two effect colorsof the interference pigments, for example red or green. Then one of thematching effect colors appears particularly intensively while the otherof the two effect colors appears as a mixed color. In addition, asubstance that absorbs light in a narrow band and does not correspond toany of the effect colors can be used for layer 11; this then producesmixed colors in both effect colors that slightly smooth the color switchbetween the two interferential effect colors, which may be desirable inindividual cases.

In order to produce the platelet-shaped interference pigments accordingto the invention in a three-layer form, the procedure is as follows:liquid crystalline main or side chain polymers or mixtures thereof areused as the liquid crystalline polymers (LCP), as well as liquidcrystalline oligomers or oligomer mixtures or liquid crystallinemonomers or monomer mixtures. These LCPs are applied in the liquid stateto a smooth substrate, for example a glass plate, a polished steelcylinder, a shiny smooth film of a plastic that has an antiadhesiveeffect with respect to the LCPs to be applied, or to a polished metalstrip, as a thin film with a thickness of about 3 to 15 μm, with themesogens of the LCPs being arranged automatically at least approximatelychirally-nematically and/or smectically and/or cholesterically. Theapplication of the film to the substrate can be performed by spreading,rolling, or spraying. A shearing process during the application of thefilm to the substrate favors a chiral-nematic or similar alignment ofthe mesogens in the film-forming polymer. The thin film thus produced iscured on the substrate. After the first film has been cured it isinitially left on the smooth substrate and a layer 11 of alight-absorbing dye is applied to this film in a layer thickness thatproduces a certain absorption effect; this layer is then cured as well.After the light-absorbent layer has cured, the two-layer sandwich isinitially left on the smooth substrate and an additional film is appliedand cured whose composition and layer thickness correspond to the firstfilm. Only then is this three-layer cured sandwich removed from thesubstrate and fractured up into particles in the shape of platelets,with lateral uncoated edges 12 resulting. After multilayer film 13 hasbeen fractured up, the particles whose diameters (L) are less than thefilm thickness (S) and the particles whose diameters (L) are more thanten times greater than film thickness (S) are sorted by a grain sizeselective separating method and the remaining particles are used asinterference pigments 8. The selected grain size spectrum of theinterference pigments is limited at the bottom end because pigments ofany size do not automatically assume the surface-parallel orientationduring paint application that is important for the direction-dependentcolor effect. The presence of a great many small pigments would alsoproduce a certain dispersion effect that would have an adverse effect onthe color brilliance of the interferentially produced colors. Thepigments must also not be too large since otherwise they would causeproblems in the equipment used for paint application and because overlylarge pigments might also possibly project out of the paint surface andhave an adverse effect on a smooth appearance of the paint surface. Thefilm fragments that are too large and are sorted out can be collectedseparately from the chosen film dust and returned to the comminutionprocess.

An alternative method for producing two- or three-layer interferencepigments consists in introducing the absorption layer as a prefabricatedfilm-type layer into the manufacturing process, applying to it and thencuring on top of it, on one or both sides, the layers that have theinterferential effect. In the case of bilateral coating of thisabsorption film, the outer coatings are time-staggered and applied oneafter the other and cured with the sandwich possibly being used afterthe application and curing of the first LCP layer so that the side ofthe sandwich that was formerly on the bottom becomes the one on top.

It is recommended to choose liquid crystalline polymers for layers 14that have an interferential effect from photo-crosslinkable orelectron-beam-crosslinkable polymers. Because of the lack of arequirement for thermal crosslinking of the LCPs on the interferencepigments, a situation can be created such that the color effect of theinterference pigments does not change in an undesirable fashion as theresult of a later baking of paintwork produced with such an effectpaint. Although in individual cases there may be LCPs that can withstandsuch a baking process with a negligibly slight color change, or theremay also be applications in which this color change can be controlled,precipitates out when processed and as a result can be kept in theinterference pigment so that thermally crosslinkable LCPs can also beallowed for the interference pigments. Usually however one will notprefer thermally crosslinking LCPs. Electron-beam-crosslinkable polymersare also of interest because they are not dependent on the presence ofcostly photoinitiators.

As for the optical and geometric quality of absorption layer 11, itincorporates certain design possibilities that will now be mentioned. Wehave already referred to the absorption effect for the entire spectrumof visible light, in other words a black absorption layer, and aspectrally-limited absorption effect that appears for a certain color ofthe light spectrum, with this color advantageously coinciding with oneof the two effect colors. An additional color-designing possibility liesin a deliberate choice of the layer thickness of absorption layer 11 sothat the degree of absorption is determined by the thickness of layer11. For example there are transparent coatings or films dyed withcertain colors which, depending on the layer thickness and/or the degreeof coloration, have a more or less pronounced light absorbent effect.Accordingly, by choosing the thickness of layer 11 or by choosing adegree of coloration of this material, the degree of absorption can bedetermined. This means that the covering power of the interferencepigments can be adjusted from completely covering to semitransparent.This possibility applies to all ranges of the spectrum, in other wordsfor black as well as colored absorption layers. When the light absorbentlayer 11 is light absorbent in the entire visible light spectrum, it isblack.

For the sake of simplicity it should be also mentioned that absorptionlayer 11 can be an evaporated or sputtered metal coating that has adefinite absorption effect in the case of very limited layerthicknesses.

The interference pigments made of several layers and produced in thisfashion can be mixed into a paint for painting articles of daily use,especially vehicle bodies. Such paints are applied to an object of dailyuse, especially a vehicle body or a part of the body in thecolor-determining cover layer 6 of paintwork 3.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. Platelet-shaped, multilayered interferencepigments for effect paint comprising at least one layer which comprisesliquid-crystal polymers whose mesogens are arranged at leastapproximately chirally-nematically, smectically, cholesterically, orcombinations thereof; and a light-absorbent layer said light-absorbentlayer absorbing at least a part of the visible light spectrum; and withthe edges of the platelet-shaped interference pigments being made in theform of circumferentially uncoated fractured edges of a multilayersandwich.
 2. Interference pigments according to claim 1, comprisingthree layers, with the light absorbent layer being located in the middleof the sandwich.
 3. Interference pigments according to claim 2, whereinthe interference pigments are built up at least approximatelysymmetrically in their layer structure with respect to the middle layer,depending on the layer thickness and the material of which the coatingis composed.
 4. Interference pigments according to claim 1, wherein thelight absorbent layer is light absorbent in the entire visible lightspectrum.
 5. Interference pigments according to claim 1, wherein thelight absorbent layer absorbs light within that partial area of thevisible light spectrum that lies outside one of the colors of theinterference pigments.
 6. Interference pigments according to claim 1,wherein the light absorbent layer is semitransparent.
 7. Interferencepigments according to claim 1, wherein the liquid crystal polymers arenot thermally crosslinking.
 8. Interference pigments according to claim1, wherein the liquid crystal polymers are photo-crosslinkable orelectron-beam crosslinkable.
 9. Interference pigments according to claim1, wherein the interference pigments have a thickness (S) ofapproximately 3 to 5 μm.
 10. Interference pigments according to claim 1,wherein the platelet-shaped interference pigments have a diameter of 5to 100 μm.
 11. Interference pigments according to claim 1, wherein theplatelet-shaped interference pigments have a diameter of 10 to 50 μm.12. A paint comprising multilayer interference pigments according toclaim
 1. 13. Paintwork applied to an object wherein thecolor-determining layer of the paintwork comprises multilayerinterference pigments according to claim
 1. 14. Paintwork according toclaim 13, wherein said object is a vehicle body or a part thereof.